Mega Evolution

Mega Evolution is a new trans-evolutionary method found native in the Kalos region. All Mega Evolutions are identified by having "Mega" in front of their name.

Method
In order to undergo a Mega Evolution, the Pokémon must be holding the applicable Mega Stone. The player must use their Mega Ring, which contains a Key Stone, to resonate with the Mega Stone held by their Pokémon to trigger Mega Evolution. Mega Evolution can only occur in battle and disappears after the battle is over. Mega Evolution is only limited to one Pokémon per battle.

X and Y
The Tower of Mastery is also related to Mega Evolution and Korrina knows secrets about Mega Evolution.

Omega Ruby and Alpha Sapphire
In Sky Pillar the Legendary Pokémon Rayquaza holds the secrets about Mega Evolution.